For many years, horticulturists and agronomists have subscribed to the belief that foliar feeding of plant nutrients is inefficient and only useful for a specific minor element deficiency as determined by tissue test of plant foliage or leaf petioles.
Dr. H. B. Tukey, plant researcher and Head of Michigan State University's Department of Horticulture in the 1950s, working with research colleague S. H. Wittwer, first proved conclusively that foliar feeding of plant nutrients really works. Researching possible peaceful uses of atomic energy in agriculture, they used radio-active phosphorous and radio-potassium to spray plants, then measured with a Geiger counter, the absorption, movement and utilization of these and many other nutrients within plants. They found plant nutrients surprisingly moved at the rate of about one foot per hour to all parts of the plants. Comparing efficiency of plant use of foliar-fed nutrients versus soil-applied nutrients near roots, they found foliar feeding provided about 95 percent efficiency of use compared to about 10 percent of use from soil applications. See Tukey, H. B. and Wittwer, S. H., 1956. The entry of nutrients into plants through stem, leaf and fruit, as indicated by radioactive isotopes. Progress in Nuclear Energy Biological Sciences Scries Six, pp. 106-114. McGraw-Hill. New York and Permagon Press, London.
Speed of absorption and use by foliar applications proved immediate, whereas with soil applications absorption and plant use both were very slow.
Commercial agricultural chemists began developing foliar feeding formulations a decade ago. Their continuous product improvement research has resulted in products containing not only specific plant nutrients, but also natural plant sugars that aid rapid entry and movement into and through plants, plus cytokinins: natural plant growth hormones extracted from seaweed. Together with nutrients, they aid natural plant defense mechanisms in resisting many plant diseases and insect pests. Healthier plants, like humans, are better able to resist many pests compared to those in stressed, poor condition. Also, growers know and observe that the weakest plants are the ones most often attacked by insect, disease and mite pests. Such products can help improve plants' health for higher yields with lower pest control inputs and plant nutrients costs. A relatively small amount of plant nutrients, foliar-applied, can replace a much greater amount that is soil-applied, and is immediately available to plants.
The development of a low-cost, natural soybean oil-based adjuvant for use with such foliar-applied nutrients and crop protectants further improved leaf and stem coverage and retention. An example is foliar application of (or even to twigs and stems after leaf drop) potassium to benefit berry, grape and tree fruits plants in late fall/early winter, or during winter in milder areas when applied anytime temperatures are above freezing. To toughen/harden plant cells, many farmers apply one gallon per acre of foliar-formulated potassium. In two weeks, a second spray of two gallons of foliar K per acre with 1 pint/acre of the soybean oil is applied. The application adds relatively minimal cost, is rain-fast in 15 minutes and is great insurance at very low cost for high-value horticultural crops. Shortcomings of this type of formulation is that it easily clogs sprayers and drip irrigators and has a short shelf life. Furthermore, it must be applied wet.